Polymerization of vinyl aromatic compounds

ABSTRACT

A PROCESS FOR THE ANIONIC POLYMERIZATION OF AT LEAST ONE VINYL AROMATIC MONOMER SELECTED FROM STYRENE AND HYDROCARBYL SUBSTITUTED STYRENES COMPRISING POLYMERIZING THE MONOMER IN THE PRESENCE OF A CATALYST COMPRISING A METAL OF GROUP 1-A OF THE PERIODIC TABLE OR A HYDRIDE OR HYDROCARBYL THEREOF CHARACTERIZED IN THAT THE FEEDSTOCK FOR THE POLYMERIZATION IS THE HYDROCARBON EFFLUENT OF A DEHYDROGENATION REACTION COMPRISING AT LEAST 5% BY WEIGHT OF SAID MONOMER AND IN THAT AT LEAT A PROPORTION OF THE HEAT OF POLYMERIZATIOM IS RETAINED IN THE REACTION MIXTURE WHEREBY THE TEMPERATURE OF THE MIXTURE IS CAUSED TO RISE AND IS FOR AT LEAST PART OF THE REACTION TIME IN EXCESS OF 100*C.

May 21, 1974 A. BENNETT- POLYMERIZATION OF VINYL ARQMATIC COMPOUNDS Filed Nov. 30, 1971 2 Sheets-Sheet l o m o w 0 c E R U m R E P M E T N m T A w m m .o 2 u 0 9 7 a s 3 2 r o o w o o o o o o o O O O O O O O O O O u AE QE 5536 MS;

May 21, 1,74 BENNETT 3,812,088

POLYMERIZATION OF VINYL AROMATIC COMPOUNDS Filed Nov. 50, 1971 2 Sheets-Sheet a F/GZ.

CATALYST MONOM ER .U.S. Cl. 26093.5 R

United States Patent 3,812,088 POLYMERIZATION OF VINYL AROMATIC COMPOUNDS Donald Alfred Bennett, Beaulieu, England, assignor to The International Synthetic Rubber Company Limited, Southampton, England Filed Nov. 30, 1971, Ser. No. 203,292 Claims priority, application Great Britain, Dec. 10, 1970, 58,757/ 70 Int. Cl. C08f 7/02, 7/04 24 Claims ABSTRACT OF THE DISCLOSURE A process for the anionic polymerization of at least one vinyl aromatic monomer selected from styrene and hydrocarbyl substituted styrenes comprising polymerizing the monomer in the presence of a catalyst comprising a metal of Group 1-A of the Periodic Table or a hydride or hydrocarbyl thereof characterized in that the feedstock for the polymerization is the hydrocarbon effluent of a dehydrogenation reaction comprising at least by Weight of said monomer and in that at least a proportion of the heat of polymerization is retained in the reaction mixture whereby the temperature of the mixture is caused to rise and is for at least part of the reaction time in excess of 100 C.

This invention relates to the polymerization of vinyl aromatic compounds.

Methods for polymerizing vinyl aromatic compounds, i.e. those containing a single vinyl group directly attached to an aromatic nucleus, which compounds may be subst-ituted with e.g. halogen or hydrocarbyl groups, are well known. These methods including bulk, bead, or emulsion polymerizations using free radical catalysts and isothermal polymerization in hydrocarbon solution employing an anionic catalyst system such as lithium alkyl. In the preparation of Winyl aromatic polymers by such methods the monomer is generally required in a substantially pure form. The vinyl aromatic monomer is usually prepared by dehydrogenation of its .alkyl aromatic precursor (e.g. styrene is prepared by dehydrogenation of ethyl benzene). Subsequent separation of the vinyl aromatic monomer from other dehydrogenation products and also unconverted precursor in the efiluent of the reaction has generally been required.

In prior art processes the control of temperature plays an important part in the production of a satisfactory end product. Thus when using an anionic system in a solution polymerization, the temperature has been kept low, typically from 30 C. to 70 C., notwithstanding reduced reaction rates,to avoid undesirable decrease in the molecular weight of the polymer product and to avoid any other complications occurring when using high temperature e.g. low yield conversion. The deleterious effects of increasing the temperature in such anionic polymerizations have been described by E. W. Duck, M. N. 'Ihornber and L. V. J. Saunders (Rubber and Plastics Age, 1964, vol. 45, pages 274 and 276).

For example, U.K. Specification No. 842,665, describes and claims a method of making polystyrene which comprises preparing a solution of styrene in an inert hydrocarbon having a boiling range close to that of styrene, polymerizing the styrene in said solution at a temperature in the range of about 40 to 100 C. in the presence of a dispersed sodium catalyst of small particle size, controlling the temperature in the polymerization step by boiling the solution at a controlled presure, removing sodium from the polymer solution and finally removing solvent from said solution. One embodiment of the process in- 3,812,088 Patented May 21, 1974 "ice eludes the step of preparing the styrene solution by dehydrogenating ethylbenzene contained in a C aromatic stream obtained from hydroformed naphtha and substantially freeing the resulting solution of styrene monomer from tar and water.

We have now found that if, contrary to previous practice in the anionic polymerization of vinyl aromatic compounds, the temperature of the reaction mixture is allowed to rise in an uncontrolled manner to levels well above those previously considered practicable, it is possible to obtain useful products of high molecular weight. In particular more rapid production of high molecular weight vinyl aromatic polymers by anionically polymerizing the monomers unseparated from the eflluent of a dehydrogenation reaction becomes feasible.

According to the present invention there is provided a process for the anionic polymerization of at least one vinyl aromatic monomer selected from styrene and hydrocarbyl substituted styrenes comprising polymerizing the monomer in the presence of a catalyst comprising a metal of Group lA of the Periodic Table or a hydride or hydrocarbyl thereof characterized in that the feedstock for the polymerization is the hydrocarbon effiuent of a dehydrogenation reaction comprising at least 5% by weight of said monomer and in that at least a proportion of the heat of polymerization is retained is the reaction mixture whereby the temperature of the mixture is caused to rise and is for at least part of the reaction time in excess of 100 C.

Preferably 50% or more of the heat of polymerization is retained in the reaction mixture.

Also according to the present invention there is pro- 9 vided an anionic polymerization process, preferably a continuous process, as just described, in which at least of the heat of reaction is retained in the reaction mixture. The reaction may be performed under substantially adiabatic conditions. Preferably a polymer is recovered having an inherent viscosity (measured under the conditions hereafter described) of at least 0.5.

The term inherent viscosity whenever used in this Specification refers to the inherent viscosity of an approximately 0.1% w./v. solution in laboratory grade toluene as determined in an Ostwald viscometer at 25 C.

In yet a further aspect of this invention there is provided an anionic polymerization process, which is preferably continuous, in which the temperature of the reaction mixture is in excess of C. for at least halfof the time of the polymerization reaction, in which the conversion of monomer to polymer is at least 80%, and in which a polymer having an inherent viscosity of at least 0.65 is recovered.

The increase in temperature (At) of the reaction mixture on reaction is primarily dependent on the heat of polymerization and the concentration of the vinyl aromatic compound, the other hydrocarbons in the mixture having a lesser effect.

The vinyl aromatic compound may be polymerized substantially in the form of the effiuent of a dehydrogenation reaction. For instance in the case of styrene as monomer, the styrene is polymerized as a solution in ethyl benzene (generally also containing a small percentage of toluene, benzene and other homologues), which is obtained directly by the dehydrogenation of ethyl benzene or of an ethyl benzene/xylene mixture such as is produced in the hydroforming of naphtha. Thus no separation of the monomer from the hydrocarbon efiluent of the dehydrogenation reaction is required before the efiluent is employed as feedstock for the polymerization.

Likewise alpha methyl styrene may be obtained prior to separation as a mixture with isopropyl benzene and may be polymerized according to the invention in this form. The concentration of vinyl aromatic monomer in the dehydrogenation efiluent may be 5% to e.g. 90% or more by weight, concentrations of 30% to 60% usually being obtained. It is important that the feedstock prior to polymerization is substantially free of air, oxygen and moisture and other impurities which react with the polymerization catalyst or otherwise interfere with the polymerization reaction. If necessary a separate purification stage may be required to remove such impurities, and references in this Specification to monomers substantially in the form of the effluent of a dehydrogenation reaction are intended to include efiluents which have been subjected to such a purification stage. Excess catalyst over that required for polymerization may be used to react with the impurities but this is expensive and wasteful.

Polymerization is initiated by means of an anionic catalyst comprising a metal of Group 1-A of the Periodic Table, or a hydride or hydrocarbyl of such a metal. Examples of suitable metals are lithium, sodium and potassium. Lithium (and hydrides or hydrocarbyls, e.g. having 1 to 6 carbon atoms, thereof) are preferred. Examples of suitable hydrocarbyl radicals are alkyl (including cycloalkyl), aryl and aralkyl radicals. Particular metal hydrocarbyl catalysts are methyl lithium, ethyl lithium, butyl lithium, amyl lithium, sodium naphthalene, benzyl lithium and phenyl lithium. Lithium alkyls, especially lithium butyl, are preferred. The amount of catalyst required is generally 5 to 15 mmoles per litre of vinyl aromatic monomer, but may be as low as about 2.5 mmoles per litre. The catalyst may be employed in the form of a solution in a suitable solvent, e.g. hexane.

Thus in the case of a lithium butyl polymerization of styrene, the lithium butyl concentration is usually in the range 0.035 to 0.11 parts per hundred parts of styrene, the minimum practicable level being about 0.020 parts. These amounts refer to active catalyst i.e. catalyst actually used to initiate polymerization. Where impurities are present in the monomer feed, some catalyst is used up in scavenging reactions and is not therefore available to initiate polymerization.

In general the inherent viscosity of the final polymer varies in inverse proportion with the catalyst concentration. Thus by varying catalyst concentration and initiation temperature, polymers having a variety of inherent viscosities may be obtained. The inherent viscosity of the product is desirably at least 0.5, preferably 0.65 to 1.4.

The initiation temperature of the mixture is chosen such that a sustained polymerization rate is obtained when the mixture is contacted with the catalyst and in general temperatures above 120 C. are not required. Typically, initiation temperatures in the range 30 C. to 100 C., e.g. 40 C. to 90 C., preferably 40 C. to 70 C., are used, initiation temperatures below about 15 C. giving rise to very slow reaction. Generally some reduction in the inherent viscosity of the final polymer is obtained as the initiation temperature is increased and by this means some control of the inherent viscosity of the final polymer may be obtained.

Once polymerization commences, the temperature of the reaction mixture rises since a proportion, preferably 50% or more, of the heat of polymerization is retained in the reaction mixture. Desirably at least 80%, and more preferably substantially all, of the heat of polymerization is retained in the mixture. The heat of polymerization of a vinyl aromatic monomer may be measured experimentally and in the case of styrene, figures have been quoted ranging from 16.1 to 17.7 kcal. per mole depending on the method of determination used. 16.7 kcal. per mole is a generally accepted value and references herein to percentage figures of the heat of polymerization may be converted to absolute values using this figure for the conversion factor. Thus from knowing the temperature rise in any given reaction and the relevant specific heats of the components of the reaction mixture, the amount of heat retained in the reaction mixture and hence the Propcrtion of the theoretical amount of heat produced may readily be calculated.

The temperature rise is such that the temperature of the reaction mixture is in excess of C. for at least a part of the reaction time and preferably for at least half of the reaction time. Temperatures of for example 200 C. or more may be attained towards the end of the reaction. Preferably the reaction, conditions are such that the polymerization may be completed rapidly. The reaction time is preferably less than 1 hour, more preferably 30 minutes or less. At for the reaction mixture is preferably at least l00.C.

The product at the end of reaction is at a temperature of 100. C. or more, preferably 180 C..or more. At the preferred final temperatures of 180 or more, the polymer product is readily devolatilized without requiring the addition of the large quantities of heat which are required in devolatilization processes-in which the polymer is prepared by conventional processes involving the use of much lower polymerization temperatures. A further advantage is that the product is molten and is thus in anideal form for further processing, e.g. pelletizing or moulding.

As mentioned above, the inherent viscosity of the polymer obtained may be varied somewhat by varying the initiation temperature and active catalyst level. In general satisfactory results may be obtained by using an appropriate combination of these parameters within the area of ABC of the graph shown in FIG. 1 of the accompanying drawings. More specifically, products of commercially desirable inherent viscosity are most conveniently obtained by selecting the parameters to be within the area DEFGH of the graph.

The monomers used in the polymerization process of the present invention are styrene, and hydrocarbyl substituted styrenes, i.e. styrene substituted in the benzene nucleus or in the vinyl side-chain by a hydrocarbon group, e.g. a hydrocarbon group having 1 to 4 carbon atoms. Examples of suitable hydrocarbyl substituted styrenes are alpha methyl styrene, vinyl toluene, and homologues of such compounds. Styrene is commercially manufactured on a large scale by dehydrogenation of ethyl benzene as indicated above and the process of the present invention permits the production of commercially valuable styrene polymer without the need for expensive separation of the styrene from the ethyl benzene.

The polymerization reaction is conveniently carried out as a continuous process, for example in a tube reactor such as an insulated pipe. However a batch process may be used and any pressure vessel in which the heat of polymerization can be readily retained may be employed. Preferably a pressure is applied to the reaction system such that all reactants remain in the liquid phase, e.g. a pressure of at least 50 pounds per square inch (p.s.i.g.), typically 100 to 200 p.s.i.g. If desired some proportion of the vapor, especially of hydrocarbons other than mono mer, may be removed from the reaction system. A continuous process is preferred and one suitable form of ap paratus for carrying out such a process is illustrated in FIG. 2 of the accompanying drawings. A source 1 of vinyl aromatic compound, e.g. styrene, namely the efiiuent of a dehydrogenation reaction which produces the vinyl aromatic compound from a precursor thereof, is connected to a drier 2 leading to a preheater 3. In the preheater the feedstock is heated to the required initiation temperature before passing to the base of an upright tube reactor 4, though it will be appreciated that this step may not always be required.

Catalyst, e.g. in solution, which is kept under a blanket of, e.g. nitrogen, in a tank 5 is also passed to the base of the reactor. The wholly anionic polymerization proceeds u ninterruptedly as the monomer and catalyst pass up the reactor, .at a rate of for example one foot per minute, the temperature of the reaction mixture being monitored if desired at suitable points. The reaction mixture is passed from the top of the reactor to a heated devolatilizer 6 wherein unreactive hydrocarbons present and any unreacted monomer(s) areremoved under vacuum for recycling through an outlet 7. The. molten polymer product which is recovered is passed through a pump extruder 8 where it is formed into a plurality of strands 9. These 3. A process accordingito claim l'wherein the reaction is performed under;substantially adiabatic conditions. 4. A process according to claim 1 wherein the temperature of the re 'ction mixture is in excess of 100 C. for at least half the time of the reaction.

strands are taken through a water cooling bath 10 to a 5 5. A process according to claim 1 wherein the amount pelletizer 11 where they cut into nihs or granules. These of active catalyst is 5 to millimoles per litre of vinyl are sieved in a sieve 12 and finally passed to a storage aromatic monomer. hopper 13 6. A process according to claim 1 wherein styrene is Fillers, reinforcing agents and/or lubrication aids may 10 polymerized substantially in the form of the efiluent of be included. For example an unsaturated rubber such the dehydrogenation of ethyl benzene. as polybutadiene and/or natural rubber or similar poly- 7. A process according to claim 1 wherein the polymermers, desirably in solution in a suitable solvent may be i-zation reaction time is less than 1 hour. admixed with the monomer feed. 8. A process according to claim 7 wherein the polymer- A preferred embodiment in accordance with the present 5 ization reaction time is 30 minutes or less. invention will now be described by way of example. 9. A process according to claim 1 wherein the inherent viscosity is from 0.65 to 1.4. EXAMPLE 10. A process according to claim 1 wherein the tem- Usihg the apparatus descnhed above, a 40% 501M101 perature of the reaction mixture at initiation of polymer- (by weight) of styrene in ethyl benzene constituted by the ization is i the range 40 to 90 C. efiluent emanating from the dehydrogenation reaction of A process according to claim 1 wherein the rise in ethyl benzene, was dried over alumina and preheated to temperature of the reaction mixture is at least 0 approX S" before Passing it to a lagged 22 12. A process according to claim 1 wherein the con- 4 inch diameter vertical tube reactor. At the point where version f monomer) to Polymer is at least thesolution entered the reactor a 15% w./w. solution of 25 A process according to claim 1 wherein the active lithium butyl 1n h x e was lhlected at thh rate of 0066 catalyst concentration and the temperature of the reaction parts of llthmm b tyl P huhdhed P styrene (on a mixture at initiation are within the area ABC shown in weight basis). Separate exeperlments indicated that ap- FIG 1 of the accompanying drawings. proximately 0.041% Pa P hundred Parts of styrefle W 14. A process according to claim 1 wherein the active Present as the acme catalyst-The mlxture f mamtamed catalyst concentration and the temperature of the reunder a pr s 0f lQ- As the rose up action mixture at initiation are within the area DEFGH the fl i PQ Y P f Place t 6 strong shown in FIG. 1 of the accompanying drawings. Qxothermlc rfleactlon Pausmg the mlxiure temperanire to 15. A process according to claim 1 wherein the process rise. The residence time was approximately 39 minutes is continuous. gggvg sog g s t xf r g l g g i f zfig g s g JPzg Eh ET; 16. A process according to claim 15 wherein the process 96%. The mixture was passed to a devolatilizer where ii iig i out m one or more thermally Insulated tube under a high vacuum and at 240 C. the residual styrene A process for the manufacture of polystyrene and ethyl benzene were removed. The molten polystyrene cording to claim 1 wherein the h d b m was removed by a pump and forced through an ex- 40 y meat on e 15 continuously supplied to the base of a tubular reactor, the truslon he and rewveredlithium alk 1 is in'ected t th r d The inherent viscosity of the product was 0189. Other base d 1 m o d 6 Imxture at Sal physical properties were measured and are given in the an e Po ys yrelze pro uct 18 i at a following Tab1e perature of at least 180 at the top of sald e t Taking the heat of polymerization of styrene as 16.7 pfocess F Q clam 17 wherein the kilocalories per mole and the specific heat of the mixture Polymenzatlon reactlonflme than 1 hour- (assumed to be constant at all temperatures) as 0.5, the A Process accordlhg to claim 17 wherein the theoretical temperature rise can be calculated to be 128 Verslon of monomer to P y at least C. The observed temperature rise was 130 C. and thus 20. A process according to claim 1 wherein the amount it can be seen that substantially all the heat of polymerizaof lithium butyl active in the p ion is 0.035 to tion was retained in the reaction mixture, i.e. the reaction 0075 parts by weight per hundred parts by weight of was performed under substantially adiabatic conditions. styrene.

TABLE Test method Test conditions R ult;

'fii fi tifiiiiifiiiiiiij: ii iiihi i -33$ -ifffffffi'.fifififf:::::::::::;::: 2 5 Flexural yield strength.- A.S.T.M. D.79066 x x 5" bar C.H.S. 0.5 cm./min 1,100 kg./cm 1 Notched impact strength A.S.T M. D.25656 (1961) x 34 x 2% bar, 25 C kg./cm./cm. F1exuralmodulus A-S- 11790-65 X 56" bar X104 lrgJcm. Rockwell Hardness.-. A.S. M. D.785-65 R & M scale W 0, 25 C 115 M 70;. Heat distortion temperature- A.S.T.M. D.64856 (1961) 66 p.s.i 945 0 Melt flow index A.S.'1.M. D.1238-65T 190 0J5 kg 1.49 g./l0 min.

What is claimed is:

1. A process for the manufacture of polystyrene having an inherent viscosity of at least 0.5 from the hydrocarbon efiluent obtained by the dehydrogenation of ethyl benzene, said efiluent containing 30% to by weight of styrene, comprising polymerizing the styrene with a catalyst consisting essentially of a lithium alkyl and retaining in the reaction mixture at least 50% of the heat of polymerization whereby the temperature of the mixture is caused to rise and is in excess of 100 C. for at least half the reaction time.

2. A process according to claim 1 wherein at least 80% of the heat of polymerization is retained in the reaction mixture.

21. A process for the anionic polymerization of at least one vinyl aromatic monomer selected from styrene and hydrocarbyl substituted styrenes comprising polymerizing the monomer in the presence of a catalyst consisting essentially of lithium or a hydride or hydrocarbyl thereof characterized in that the feedstock for the polymerization is the hydrocarbon efiluent obtained by the dehydrogenation of a hydrocarbon stream comprising the alkyl benzene precursor of the vinyl aromatic monomer, said feedstock comprising 30 to 60% by weight of said monomer and in that at least 50% of the heat of polymerization is retained in the reaction mixture whereby the temperature of the mixture is caused to rise and is for at least part of the reaction time in excess of 100 C. and maintaining References Cited the reaction conditions such that the inherent viscosity of UNITED STATES PATENTS the polymerp roduct after recovery is at least 0.5. 2,813,137 [11/1957 Twaddle 260 93 5 22. A process according to claim 21 wherein the catalyst 3,110,705 11 19 3 v u 2 0 94 2 is lithium metal, or a lithium alkyl, aryl, or aralkyl. 5 3,248,377 4/ 1966 Arnold 26093.5

23. A process according to claim 21 wherein the cata- 3,560,469 2/1971 PICPYS et a1 260 93-5 W is lithium JAMES A. SEIDLECK, Primary Examiner 24. A process according to claim 21 wherein the feedstock consists essentially of ethyl benzene or isopropyl 10 l- X-R- benzenm 260-4 AR, 88.2 C, 880 R 

